1. Field of the Invention
This invention relates generally to sheet-fed or web-fed rotary offset lithographic printing presses, and more particularly, to an improved printing and coating apparatus which combines lithographic and flexographic printing and coating operations and methods in a continuous in-line operation without the loss of a printing unit.
2. Background of the Prior Art
Offset lithography is a process well known in the art which utilizes the planographic method. Image and non-printing areas are essentially on the same plane of a thin metal plate and the distinction between them is maintain chemically. Ink is offset from a plate on the plate cylinder to a rubber blanket on a blanket cylinder and then from the blanket to a substrate supported on an impression cylinder on which printing occurs.
Conventional sheet-fed, rotary offset printing presses typically include one or more printing units through which individual sheets are fed and printed. After the last printing unit, freshly printed sheets are transferred by a delivery conveyor to the delivery end of the press where they are collected and stacked uniformly. In a typical sheet-fed, rotary offset printing press such as the Heidelberg Speedmaster line of presses, the delivery conveyor includes endless chains carrying gripper bars with gripper fingers which grip and pull freshly printed sheets from the last impression cylinder and convey them to the sheet delivery stacker.
Printed lithographic ink on the surface of the substrate sheet dries relatively slowly through oxidation and is easily smeared by subsequent transfer cylinders between the individual printing units of the press. Any relative movement of the freshly printed surface relative to a support surface can result in smearing. Modified and specialized equipment and techniques have been developed to combat this problem.
A related problem that is faced in the prior art is the problem of "offsetting" and "set off" of freshly printed ink at the delivery end of the press after the printed sheets are collected and stacked. A similar problem occurs in roll form material produced on a web-fed press. In some printing jobs, offsetting is prevented by applying a protective and/or decorative coating material over all or a portion of the freshly printed sheets. Some coatings are formed of an ultra-violet (UV)-curable or water-dispersed resin applied as a liquid solution over the freshly printed sheets to protect the ink from offsetting or set-off and improve the appearance of the freshly printed sheets. Such coatings are particularly desirable when decorative or protective finishes are applied in the printing of posters, record jackets, brochures, magazines, folding cartons and the like. In cases where coating is to be applied, the coating operation is carried out after the last printing unit, most desirably by an in-line coating application. It is highly undesirable to process the sheet through the press a second time in order to apply coatings, although this is sometimes done for special effects that are not otherwise obtainable.
The ability to overall coat, spot coat or print with aqueous, flexographic and UV curable inks and/or coatings in combination with lithographic, flexographic and waterless printing processes on a rotary offset printing press is highly desirable. Flexographic printing or coating with aqueous, flexographic and UV curable inks from a blanket or a relief plate can permit much heavier wet and dried ink film layers on the substrate. This is largely due to the nature of lithographic inks. Lithographic inks are generally oil based inks that are formulated to print from planographic surfaces based on the principle that oil and water do not mix. Lithographic inks are generally very strong in color value to compensate for the lesser amount that is printed. They are among the strongest of all inks. The average amount of ink transferred to the paper is further diluted by the double split of the ink film between the plate cylinder and the blanket cylinder and between the blanket cylinder and the substrate to be printed in the nip between the blanket cylinder and the impression cylinder. In many situations, only a quarter of the film thickness on the plate is transferred to the substrate. This can make it difficult to obtain sufficient opacity with white or metallic (gold, silver or other metallic) ink or in printing specialized vehicles such as "scratch-and-sniff" materials from a slurry containing encapsulated essence. This often mean that sheets or substrate must be removed and transferred to a second type of machine using the flexographic process to apply greater amounts of ink thickness and make successive or two passes to achieve desired print quality.
The prior art has attempted to solve these problems and obtain higher applied film weights in a variety of ways. It is known to provide a printing machine with a downstream coating station having a blanket roller coater associated with a coating application unit for application of a protective coating over the entire printed area of copy sheets or web before they go to the stacker. Jahn, U.S. Pat. Nos. 4,615,293 and 4,706,601 disclose separate duplex coating units disposed downstream of a printing press. These permit coating selected portions of the substrate using a relief plate and they permit blanket coating.
A number of coating units are known which are appended to or mounted upon the final printing station in the press. Most of these coating units prevent of the printing unit on which it is mounted from doing its normal printing function resulting in the loss of one printed color. A four color press using such a coating apparatus would permit printing only three colors in line in a single pass operation because the last station is converted to a flexographic printer-coater. Bird U.S. Pat. Nos. 4,796,556 and 4,841,903 disclose a liquid application station for the final downstream printing station which converts the lithographic station to a printing coater or a continuous film coater by moving a carriage having a coating unit into impression with the plate or blanket cylinder of the last station on the press. When the coater is used, the normal lithographic printing function on that station is inoperative. DiRico, U.S. Pat. No. 4,685,414 discloses a process and apparatus in use in combination with the last station of an existing offset lithographic press where the coating means is retractable to be used or not as a printer requires. Since the DiRico coater utilizes a blanket cylinder on the last unit of the press, this last unit cannot be used for color printing when it is used for coating. DeMoore, et al., U.S. Pat. No. 5,651,316 discloses a retractable printer-coater unit which though not limited to the last printing station of an offset lithographic press, is useful for lithographic or flexographic printing when the ordinary lithographic operation of the station in which it is mounted is not being used. The lithographic operation of the station is lost when this printer-coater is in operation. Sarda, U.S. Pat. No. 4,889,051 illustrates a retractable lithographic printing unit which does not disable normal lithographic printing on the lithographic printing station. It enables printing another lithographic color at a station by adding a second blanket roller and a retracting inked and dampened applicator for the second blanket roller of the printing station.
Koehler, et al., U.S. Pat. Nos. 4,934,305 and 5,178,678 disclose a flexographic liquid film applicator unit which employs a special "blanket" cylinder which engages the substrate on the impression cylinder of the last lithographic printing station on a multi-color lithographic press. The unit slides in and out on "inclined tracks". Manual reengagement and registration of a drive gear on the applicator with a press drive gear using "index" marks is required to reset the applicator "blanket" cylinder after the unit has been moved away from the printing station.
DeMoore, et al., U.S. Pat. No. 5,176,077 is a delivery cylinder coater for use on the final printing station of a lithographic printing press. The delivery cylinder is provided with a coating blanket. A flexographic applicator roll applies liquid coating to the delivery cylinder as it rotates into the freshly lithographically inked surface of the sheets coming off the press. The coating pickup anilox roller frictionally engages the surface of the delivery cylinder and is rotated by a hydraulic motor.
Such apparatus provided by the prior art has a number of advantages and disadvantages. Retrofitting existing presses is often difficult because of space considerations. A dedicated coating unit is often not possible because of space considerations and involves press downtime and substantial capital costs. Devices that utilize the print cylinder or blanket cylinder of the press limit the ability of that station to lithographically print in the normal manner. In retrofitting presses, space considerations are a major problem, especially the space between printing units.
Coaters which utilize the plate cylinder or the blanket cylinder of the printing station still suffer from the disadvantage that the coating is split which reduces the wet film thickness that can be applied to the substrate itself. A few add on coating units that print directly on the substrate on the impressions cylinder or a transfer cylinder are limited to the last printing station on the press where there is more room for installation. Such equipment can be moved away or the operator can do the make ready work on the opposite side of the last printing station in the conventional work space for the operator. If such equipment is mounted in the interstation space on a lithographic press, the equipment interferes with operator access to the next station.
It is preferable not to have to cut into press frame to gain access to the main gears and not to have to manually engage and disengage indexed gear teeth of gears on the coater with gears on the press. The ability to flexographically coat, spot coat or print on the substrate at an intermediate printing station with an apparatus that parks completely out of the way, which permits coating on the substrate just after it has been lithographically printed on the same impression cylinder, can apply a full coating thickness directly to the substrate, does not require modifying the press drive or cutting into the press gears, and permits further lithographic printing or perfecting on the next or following stations would be highly desirable. The present invention is able to fulfill these needs and more.